My Amp is Broken - Not Sure What to Do - Any Recommendations?

The parts did not go obsolete.  Check OnSemi, aka ON Semiconductor who manufactures the parts.  Audio Research EOLed the SD135 and HD220.  

johnsonwu, cool!

Hey all,

Thank you for the replies and helpful advice. The fellow I spoke with at Audio Research told me the Thermal Trak transistors currently offered by OnSemi are a new version of the ones used in my SD135. He said that unfortunately they are not the same as what was used in the SD135 and (according to him) will not work in my unit. 

The guy I spoke with at ARC told me that the company considered trying to come up with a work around for the original Thermal Trak transistors no longer being available, but nothing ever materialized.

I suspect a lot of people are going to be having difficulty with their amps over the next decade or so when the Thermal Trak transistors fail on them. These particular parts are known to have a somewhat significant rate of failure. Besides Audio Research, McIntosh used them, as well as Arcam, Mark Levinson and Ayre.

https://www.diyaudio.com/forums/solid-state/237929-thermaltrak-transistors-discontinued.html

I believe some manufacturers are still using the latest versions of Thermal Trak transistors in their current product lines. Although it looks like Ayre just stopped using them this year: https://www.audioasylum.com/cgi/vt.mpl?f=amp&m=184484

Anyway, this really stinks. I never would have thought this would have happened when I purchased my SD135. I may be wrong, but I think if Bill Johnson were still alive, ARC would come up with the work around to service the discontinued gear of his loyal customers.

https://www.mouser.com/ProductDetail/ON-Semiconductor/NJL3281DG?qs=sGAEpiMZZMvFp%252byPHbnZYyVkaBWrH...

3281D was INDEED obsolete.
Original collateral dated June 2006
http://www.onsemi.com/pub/Collateral/NJL3281D-D.PDF

replaced by 3281DG  Same application notes per Mouser.com and ONSemi.  Available and NOT discontinued.

My HD220 has not blown (yet) so I have not attempted a repair myself.
I suspect it means you need to change out ALL the devices for matching sake.  That's likely why Audio Research doesnt want to deal with it.
Boy if they could just respond to my email and get me the schematic!

It was my experience, that William Z Johnson cared very much, about his customers and his corporation. On the other hand; I’ve no doubt Macintosh wishes the corporation(and the competition) never existed, though they now own it.